Is a possible extra dose of RotaTeq safe?

My son is 61/2 months old.  At his 2 month well baby he was given his first dose of RotaTeq along with his other recommended vaccines.  At his 4 month well baby we were under the impression that he got everything he needed, but when we went back in for his 6 month well baby appointment and vaccines, they discovered that they didn't have record of if he got the 2nd dose of RotaTeq at his 4 month appointment.  That had dated everything else he got that day, but the RotaTeq was not listed.  So, they have no idea if he was given the 2nd dose of this vaccine at his 4 month appointment since it is not listed anywhere in his record.  It shows that the Dr. ordered it, but no record of if the nurse administered it.  My husband and I both are trying to remember if he got it, but we can't remember for sure.  They gave him a dose at that 6 month appointment and the Dr. said that we needed to come back in 4 weeks to give him another dose to make sure he will have gotten 3 doses in before he reaches 32 weeks old.  My question is:  Is it safe for him to have this extra dose if he did happen to get the 2nd dose at 4 months?  If he did have 3 already he would be given a fourth.  If he missed the 2nd, this would be his 3rd dose.  Also, is it safe that he will be given it only 4 weeks from the last dose?  I just worry about giving my baby a possibley un-needed extra vaccine.
My Dr. says it is better to maybe have an extra dose than to be under vaccinated if the 2nd dose was missed.  
I would appreciate any advise on this issue!  Thanks so much!